Prior applications such as French Patent Publication No. 2568519 and European Patent Publication No. 291 217 propose using wheel speed signals from the vehicle wheels such as for example the signals from an anti-lock braking system which are multi-pulse signals of typically 48 to 96 pulses per revolution of each wheel. The prior art system compares the speed derived signals in various ways, and also attempts to overcome errors due to vehicle factors such as cornering, braking, accelerating, uneven or changing loads, etc., which can cause changes in the speed signals which are larger than those caused by a tire deflation of, for example, 0.4 bar.
French Patent Publication No. 2568519 avoided errors of this type by monitoring the speeds of the diagonally opposed pairs of wheels for a long time or distance period so that it averaged out effectively cornering of the vehicle. The result however was that the device operated very slowly taking many kilometers to sense a pressure loss.
European Patent Publication No. 291 217 improved the situation by calculating the lateral and longitudinal acceleration of the vehicle using the same four wheel speed signals and setting fixed limits above which the detection system was inhibited to avoid false signals due to cornering and acceleration. This inhibition of detection however meant that for a proportion of the time of vehicle running the system was not sensing punctures, the actual proportion depending upon the type of roads and the way the vehicle was being driven.
The real difficulty with these types of systems is that, apart from the lateral acceleration of the vehicle which occurs during cornering causing increased deflection of the outer pair of wheels compared to the inner pair of wheels, each vehicle has different characteristics due to the position of the center of gravity and the type of suspension. These different characteristics, when concerning, produce additional deflections in the outer pairs of tires with regard to the inner pairs of tires.
However, the vehicle characteristics make the tire deflections different in each of the tires. Similar problems occur due to vehicle characteristics in the deflections in the front pair of tires compared to the rear pair tires when the vehicle brakes, and vice-versa when the vehicle accelerates.